1) I doubt the 20D will be a better camera for you. If fact, proabably quite the opposite, as you'll have more options to deal with, and the extra fps or faster shutter speeds has no use in your type of photography. Imagewise both cameras are similar.
2) I think both the Tamron 17-35 or Canon 17-40L will be better than the 28-75, because you need wideangle coverage for car interiors. All of them are very sharp lenses, and much better than the kit lens. I'm using both Tamrons as my main lenses with excellent results in my 20D.
3) The 550EX will suit your application pretty well. Probably the 420EX will be enough too, except to fill strong sunlight shadows. Guillermo..
Thanks for the response. I really appreciate it and after seeing the number of posts you have made, I know I'm getting good advice! I will take your recommendation on staying with the XT and going with either the 550EX or 420EX Flash. One clarification I'd like to get from you is concerning the lens. My experience with wideangle lenses has been that they distort shapes, which is a problem for my intent. I'm wondering if I'd be better off with a 17-35 for the interiors where the distortion isn't as critical and a 28-75 for the exterior where it is imperative to have to prevent any distortion?..
Remember the crop factor. The 17-35 is similar to a 27-56 lens in 35mm terms, so it goes from a standard wideangle to normal, so you'll have no wild perspectives asociated with ultrawideangles, mostly if you use it in the 24-35 mm range. Of course the 28-75 will give a more "flat" look to the picture, mainly at the tele end and a little sharper picture but will be too limiting for interior work because the same crop factor. So, if you can afford both lenses, go for them, but if you only can buy one, I think the 17-35 is the one. Guillermo..
I've been researching places to buy the TAMRON 17-35 DI F2.8-4 SP AF LENS(CANON) and the best price I've found has been ephotocraft.com for $409 after $40 rebate. Can anyone provide feedback on ephotocraft.com or recommend a better source/price to buy this lens?..
All the reviews I read on them look good and the price is allmost the same as Buydig.com $408 after rebate delivered which is one of the cheapest online stores I buy from. Go for it..
If your image are too much "fisheyed", You can use a plug-in in Photoshop to reshape the photo. you lose some pixels in the corner but your picture is straight..